The vapor compression air-conditioning system includes an air-conditioning/heat pump system, a refrigeration system, a heat pump system, and a heat pump water heater system. In essence, a working process of the vapor compression air-conditioning system is as follows: The refrigerant of the vapor compression air-conditioning system absorbs heat from a low-temperature medium (for example, indoor or outdoor air), and after the temperature is increased by a compressor through compression, heat is released to a high-temperature medium (for example, outdoor or indoor air). To achieve a most economical and efficient operational effect, a condensing temperature of the refrigerant of the vapor compression air-conditioning system should be higher than the temperature of the high-temperature medium by a minimum reasonable temperature difference .DELTA.Tk, while an evaporating temperature of the refrigerant should be lower than the temperature of the low-temperature medium by a minimum reasonable temperature difference .DELTA.Te.
In a design condition, the foregoing requirements may be basically fulfilled, and in this case, the system reaches an optimal energy efficiency ratio and a maximum working capability. However, in an actual operation process, the foregoing ideal requirements always cannot be met.
In the conventional air-conditioning/heat pump system, to respond to condition changes, there are mainly the following two performance adjustment means:
(1) Variable-frequency adjustment: Variable-frequency adjustment is mainly to achieve an objective of changing the flux of the refrigerant and power consumption of the compressor by changing a rotational speed of the compressor. Thereby, in a situation in which an air temperature is not high, an objective of energy saving may be achieved by property decreasing the rotational speed of the compressor.
(2) Adjustment of expansion valves, including a thermal expansion valve, an electronic expansion valve, and the like. An adjustment principle thereof is to achieve an objective of changing the flux of the refrigerant and an expansion ratio by changing the throttling area of the expansion valve.
Neither of the foregoing two adjustment means can change an average density of the refrigerant in the system, or in other words, a refrigerant charge. It is proved by both theory and experiment that in each condition, there is an optimal refrigerant charge. With this optimal charge, the system works in an optimal state, and has an optimal energy efficiency ratio. When the system deviates from the design condition, the average density of the refrigerant cannot be changed by using the foregoing two adjustment means, and therefore, the system can hardly work in a state of an optimal energy efficiency ratio (EER or COP).
When a unit switches between the refrigeration condition and the heat pump condition, the foregoing problem is especially severe. For example, in the refrigeration condition in summer, an ambient temperature range of the system is 27.degree. C. (indoors) to 35.degree. C. (outdoors); in the heat pump condition in winter, an ambient temperature range of the system is 20.degree. C. (indoors) to 2.degree. C. (outdoors). In the foregoing temperature conditions, during refrigerating in summer, the condensing temperature of the refrigerant should be set to 50.degree. C. properly, and the evaporating temperature of the refrigerant should be set to 12.degree. C.; during heating in winter, the condensing temperature of the refrigerant should be decreased to 35.degree. C., and the evaporating temperature of the refrigerant should be decreased to −13.degree. C. Apparently, in the two conditions, there is a difference of 20.degree. C. between working temperatures of the refrigerant. Because a gaseous refrigerant has different densities in different temperatures, a difference of 20.degree. C. causes a density difference of the gaseous refrigerant to be greater than 50%. That is, in the refrigeration condition and the heat pump condition, there is a great difference between optimal refrigerant charges in the system. Apparently, the conventional air-conditioning system can hardly adapt to this situation. If the refrigeration condition is considered in the charging refrigerant of the conventional air-conditioning system, then the refrigerant in the heat pump condition will be too excessive. To reduce this difference, in design of the conventional air-conditioning system, the heat exchanging area of the indoor heat exchanger is decreased intentionally, and a temperature difference of the indoor heat exchanger is increased. Thereby, during refrigerating in summer, the evaporating temperature of the refrigerant is decreased from 12.degree. C. to 5.degree. C., while during heating in winter, the condensing temperature is increased from 35.degree. C. to 43.degree. C. In this case, the difference between working temperatures of the refrigerant in the two conditions in winter and summer is reduced to approximately 3.degree. C., and the difference between optimal refrigerant charges is decreased, at a cost of a decrease of the design energy efficiency ratio, which is decreased from 4 to approximately 3.
Even during the operation in the refrigeration-only condition or the heat pump condition, the conventional air-conditioning system deviates from the design condition in most of time. The indoor temperature is decreased slowly just after the air-conditioning system is started; however, after the indoor temperature becomes stable, the outdoor temperature may change with time, and the refrigerant charge of the conventional air-conditioning system cannot be precisely adjusted accordingly. This indicates that the refrigerant charge of the conventional air-conditioning system is not optimal in most of time.